Cerium-based abrasive, production process thereof

ABSTRACT

A mixed light rare earth compound which has been obtained by chemically removing medium-to-heavy rare earth elements, Nd and impurities other than rare earth elements from an ore containing rare earth elements is fired at 500 to 1100° C. to yield a mixed rare earth oxide. A cerium-based rare earth fluoride is added to the mixed rare earth oxide to obtain a mixture. The mixture is subjected to wet-pulverization, drying, firing, disintegration and classification to thereby yield a cerium-containing abrasive.

CROSS REFERENCE TO RELATED APPLICATION

This application is an application filed under 35 U.S.C. §111(a)claiming the benefit pursuant to 35 U.S.C. §119(e) (1) of the filingdate of Provisional Application No. 60/269,843 filed Feb. 21, 2001pursuant to 35 U.S.C §111(b).

TECHNICAL FIELD

The present invention relates to a cerium-containing abrasive to be usedfor polishing materials, such as glass, and to a method for producingthe abrasive. More particularly, the invention relates to a process forproducing a cerium-containing abrasive comprising cerium oxide as apredominant component and to be used for finish-polishing ahigh-precision glass substrate, such as a hard disk substrate of glass,a glass substrate for a liquid crystal display panel, etc.

BACKGROUND ART

In recent years, glass material has found a variety of applications andhas given rise to a need for surface polishing of the material in somecases. For example, during production of optical lenses and glasssubstrates for optical lenses, the surface of glass material must bepolished to a very high degree of surface fineness so as to provide amirror surface. Particularly, flat and defect-free surfaces with minimalsurface roughness are required for glass substrates for optical andmagnetic disks, those for liquid crystal displays, such as thin-filmtransistor (TFT) type and twisted nematic (TN) type LCDs, those forcolor filters of liquid crystal television displays and those for LSIphotomasks. This brings about a demand for a surface polishing techniqueof higher precision.

High heat resistance is demanded of glass substrates for liquid crystaldisplays, since such substrates undergo high-temperaturepost-heat-treatment. In addition, substrates have become thinner andthinner in the trend toward weight reduction. Regarding a magnetic diskglass substrate, there are also demands to reduce the thickness of thesubstrate keeping with the trend in weight reduction and high mechanicalstrength, particularly rigidity, so as to withstand, for example,deformation of the disk during high-speed rotation. The levels of thesedemands have become higher year by year.

In order to meet the aforementioned demands for reduced thickness andmechanical strength, improvements have been made to the chemicalcomposition of glass and to the method for producing glass, providinguse of a glass substrate predominantly containing aluminosilicate as asubstrate for liquid crystal displays or magnetic disks. Regarding amagnetic disk glass substrate, there have conventionally been developedglass-ceramic substrates predominantly containing lithium silicate orquartz crystals as a major component. The glass of these substrates hasconsiderably poor processability, and therefore, when a conventionalabrasive is used, processing speed is low to thereby deteriorateproductivity. Thus, there are demands for a large polishing rate andhigh-precision surface polishing performance.

Regarding conventional abrasives employed for surface polishing of aglass substrate, there has been employed an abrasive predominantlycomprising rare earth oxide, inter alia cerium oxide, since cerium oxideexhibits a polishing rate several times that of iron oxide, zirconiumoxide or silicon dioxide. During use of such an abrasive, abrasivegrains are generally dispersed in liquid, such as water. When aconventional cerium-oxide-containing abrasive is used for polishing theaforementioned glass substrate of high hardness, there arises a problemof a poor polishing rate.

Although the polishing mechanism of a cerium-oxide-containing abrasivehas not been fully elucidated, it has been phenomenologically confirmedthat the polishing proceeds on the basis of synergistic effect ofchemical action of cerium oxide on glass and mechanical actionattributable to the hardness of cerium oxide particles themselves.However, since a glass substrate predominantly comprisingaluminosilicate and a glass-ceramic substrate predominantly comprisinglithium silicate have excellent resistance to chemicals, the chemicalaction of the abrasive is not fully attained. In addition, collapsing ofabrasive grains readily occurs, because of the high hardness of theseglass substrates (to be processed), resulting in failure to maintainsufficient mechanical action on glass and consequently in deteriorationof the processing rate.

In order to maintain the mechanical action for a long period of time,addition of powdered particles of, e.g., alumina or zirconia havinghigher hardness than a substrate to be processed, to an abrasivecomposition may be a conceivable approach. However, when this approachis followed, the relative concentration of cerium oxide decreases,resulting in poor chemical action. In addition, powdered particles ofhigh hardness adversely impart defects, such as pits and scratches, tothe glass surface (of the substrate to be processed).

The present invention has been accomplished so as to solve theaforementioned problem involved in conventional techniques. Thus, anobject of the invention is to provide a process for producing acerium-containing abrasive that maintains for a long period of time aninitial polishing rate relative to a glass material which is hard and isnot readily polished at a large polishing rate, and imparts no surfacedefects, such as pits and scratches, to a material to be polished, suchas glass, to thereby provide a polished surface of high quality. Anotherobject of the invention is to provide a cerium-containing abrasiveproduced through the process.

DISCLOSURE OF THE INVENTION

A cerium-containing abrasive according to the invention comprises cubiccomposite rare earth oxide and composite rare earth oxy-fluoride,wherein the abrasive contains not less than 90 mass % of rare earthelements in terms of oxide and the rare earth elements contain not lessthan 55 mass % of cerium in terms of oxide.

The abrasive includes an abrasive, when subjected to X-ray diffractionmeasurement, having a main peak resulting from the cubic composite rareearth oxide that is not less than 28.2 degrees at 2θ.

The abrasive includes an abrasive, when subjected to X-ray diffractionmeasurement, having a main peak resulting from the rare earthoxy-fluoride whose intensity ratio to a main peak resulting from thecubic composite rare earth oxide is in the range of 0.2 to 1.0.

The abrasive includes an abrasive having primary particles with aparticle size of 10 to 50 nm and a relative surface area of 2 to 10m²/g.

The abrasive includes an abrasive containing 5 to 10 mass % of fluorine.

A process for producing a cerium-containing abrasive according to theinvention comprises firing at 500 to 1100° C. a mixed light rare earthcompound which has been obtained by chemically removing medium-to-heavyrare earth elements, Nd and impurities other than rare earth elementsfrom an ore containing rare earth elements to thereby yield a mixed rareearth oxide.

The producing process includes adding a cerium-based rare earth fluorideto the mixed rare earth oxide and subjecting the resultant mixture towet-pulverization, drying, firing, disintegration and classification tothereby yield a cerium-containing abrasive.

The producing process includes adding cerium-based rare earth fluorideand mixed rare earth carbonate obtained by carbonating the mixed lightrare earth compound to the mixed rare earth oxide and subjecting theresultant mixture to wet-pulverization, drying, firing, disintegrationand classification to thereby yield a cerium-containing abrasive.

In the producing process, the rare earth fluoride is fluoride of acerium-based mixed light rare earth compound that is obtained by addingfluoride to the mixed light rare earth compound and heat-treating theresultant mixture at a temperature of not more than 400° C.

In the producing process, the firing is performed in the atmosphere for2 to 36 hours using a firing furnace, such as an electrical furnace or apusher furnace.

The cerium-containing abrasive having the constitution as describedabove can maintain for a long period of time an initial polishing raterelative to a hard glass material and provide a high-quality polishedsurface with small surface roughness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an X-ray diffraction diagram of the abrasive obtained inExample 1.

FIG. 2 is an X-ray diffraction diagram of the abrasive obtained inComparative Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

A cerium-containing abrasive according to the invention containsfluorine and specifically comprises cubic composite rare earth oxide andcomposite rare earth oxy-fluoride, wherein the abrasive contains notless than 90 mass % of rare earth elements in terms of oxide and therare earth elements contain not less than 55 mass % of cerium in termsof oxide.

The content of the fluorine in the cerium-containing abrasive ispreferably in the range of 5 to 10 mass %. When it is less than 5 mass%, the polishing rate becomes small. On the other hand, when it exceeds10 mass %, the rare earth oxy-fluoride will remain to cause a smallpolishing rate and form scratches.

When the rare earth elements are shown by RE, the cubic composite rareearth oxide is a compound represented by RE₂O₃, for example, and thecomposite rare earth oxy-fluoride is a compound represented by REOFREO,for example. The cerium-containing abrasive according to the presentinvention has to be comprised substantially of the two compounds. Whythe word “substantially” is used herein is that the cerium-containingabrasive may further contain additives etc. Ordinary X-ray diffractionmeasurement of the abrasive will detect few crystal peaks resulting fromthe substances other than the two compounds. It is specified herein thatthe crystal peaks resulting from the substances are not included in thecrystal peaks resulting from the two compounds.

It is specified herein that the content of all the rare earth elementsin the abrasive is not less than 90 mass %, preferably not less than 95mass %, in terms of oxide and that the content of cerium in all the rareearth elements is not less than 55 mass %, preferably not less than 60mass %, in terms of oxide. The content of all the rare earth elements inthe abrasive is measured by analysis with equipment, such as ICPanalysis or fluorescent X-ray analysis. The content of the rare earthelements in the abrasive is measured by analysis with equipment, and themeasured content is calculated in terms of oxide.

When the content of all the rare earth elements in the cerium-containingabrasive according to the present invention is less than 90 mass % orwhen the content of cerium in all the rare earth elements is less than55 mass %, the number of particles not attributable to polishingincreases to cause a small polishing rate and generate, duringpolishing, scratches on a material to be polished.

The cerium-containing abrasive of the present invention, when subjectedto X-ray diffraction measurement, preferably has a main peak (at 2θ)resulting from the cubic composite rare earth oxide that is not lessthan 28.2 degrees. The X-ray diffraction measurement reveals that themain peak resulting from ordinary cerium oxide appears at 27.8 degrees.The main peak in the present invention appears at a position shifted bynot less than 0.4 degree toward the high angle side. The reasontherefore is that lanthanum oxide, praseodymium oxide, etc. that arerare earth components other than cerium oxide are taken in the ceriumoxide to vary the intercrystalline distance. Use of thecerium-containing abrasive having the diffraction peak mentioned abovecan provide effects of attaining a large polishing rate and reducingscratches generated during polishing.

The X-ray diffraction measurement of the cerium-containing abrasive ofthe present invention is performed using an X-ray diffraction measuringapparatus made by Kabushiki Kaisha Rigaku under conditions of an X-raygenerating voltage of 40 kV, a current of 30 mA, a scan speed of 4.0degrees/min, a measuring step of 0.02 degree/min, a DS emanation slit of1, a RS light-receiving slit of 0.3 and an SS scattering slit of 1.

In order for the X-ray diffraction of the invention to strictlyprescribe the peak position, CuK α-ray measurement is conducted using aCu anode as an X-ray pipe bulb and Ni foil as a filter.

The intensity ratio of the main peak resulting from the rare earthoxy-fluoride to the main peak resulting from the cubic composite rareearth oxide, obtained when the cerium-containing abrasive of the presentinvention is subjected to X-ray diffraction measurement, is preferablyin the range of 0.2 to 1.0, more preferably 0.3 to 0.6. In the X-raydiffraction of a cerium-containing abrasive, the main peak resultingfrom the cubic composite rare earth oxide (at 2θ) and the main peakresulting from the rare earth oxy-fluoride appear at around 28.2 and26.7 degrees, respectively. The peak intensity means the maximum valueof the diffraction intensity. When the intensity ratio of the main peakresulting from rare earth oxy-fluoride to the main peak resulting fromthe cubic composite rare earth oxide is less than 0.2, it is impossibleto sufficiently suppress adverse affection by the lanthanum oxidecontained in the cerium containing abrasive, thereby lowering thepolishing ratio and shortening the service life of the abrasive. Whenthe intensity ratio exceeds 1.0, the number of the oxy-fluorides lackingin polishing ability increases, thereby lowering the polishing ratio.

The cerium-containing abrasive of the present invention has primaryparticles whose particle size is preferably in the range of 10 to 50 nmand whose relative surface area is preferably in the range of 2 to 10m²/g. The primary particle size is measured by calculating thecrystallite diameter read from the peak width at half height of theX-ray diffraction peak, and the relative surface area is measured by theBET method.

When the primary particle size is lower than 10 nm, the cerium oxide oroxy-fluoride is not sufficiently crystallized to lower the mechanicalpolishing power. When it exceeds 50 nm, there give rise to hard largecrystals that form scratches. When the relative surface area is lessthan 2 m²/g, scratches are formed similarly to the case where theprimary particle size exceeds 50 nm. When it exceeds 10 m²/g, thepolishing rate is lowered.

The process of producing a cerium-containing abrasive according to thepresent invention preferably includes the step of chemically removingcomponents other than rare earth elements that include alkali metal,alkaline earth metal and radioactive substances, and rare earthcomponents that include medium-to-heavy rare earth elements and Nd froman ore (a rare earth concentrate) containing large quantities ofnaturally occurring cerium (Ce), lanthanum (La), praseodymium (Pr),neodymium (Nd), etc. to thereby yield, as a primary raw material, acerium-based mixed light rare earth compound, such as mixed rare earthcarbonate or mixed rare earth hydroxide, in which the amounts of thesecomponents have been reduced and the step of firing the primary rawmaterial at a temperature of 500 to 1100° C. to yield a mixed rare earthoxide. The term “medium-to-heavy rare earth elements” used herein refersto rare earth elements having an atomic number greater than that of Pm(promethium).

As the method of chemically removing components other than rare earthelements that include alkali metal, alkaline earth metal and radioactivesubstances, the method of roasting a rare earth concentrate withsulfuric acid is generally used. As the method of chemically removingthe rare earth components that include medium-to-heavy rare earthelements and Nd, the solvent extracting method is generally used.

The mixed light rare earth compound that is the primary raw materialemployed in the present invention can be obtained, for example, bypulverizing a rare earth concentrate containing large quantities ofnaturally occurring cerium, lanthanum, praseodymium, neodymium, etc.,then roasting the pulverized rare earth concentrate with sulfuric acidand dissolving the same in water, subsequently removing components otherthan rare earth elements, such as alkali metal, alkaline earth metal andradioactive substances as insoluble substances, chemically removing rareearth components including medium-to-heavy rare earth elements and Ndusing the solvent extraction method, and converting the resultant to acarbonate salt by use of ammonium bicarbonate or oxalic acid. The mixedlight rare earth compound thus obtained preferably contains 45 to 55mass % of rare earth elements in terms of oxide, and the rare earthelements preferably contain 55 to 63 mass % of cerium oxide, not morethan 0.5 mass % of non-rare earth elements and the rest of carbonicacid.

The process of producing a cerium-containing abrasive according to thepresent invention includes the steps of adding cerium-based rare earthoxy-fluoride to the mixed rare earth oxide obtained by firing, andsubjecting the resultant mixture to wet-pulverization, drying, firing,disintegration and classification.

The cerium-based rare earth fluoride is preferably obtained bychemically removing components other than rare earth elements, such asalkali metal, alkaline earth metal and radioactive substances and whennecessary in addition thereto medium-to-heavy rare earth elements and Ndfrom a rare earth concentrate containing large quantities of naturallyoccurring cerium, lanthanum, praseodymium, neodymium, etc. to provide amixed light rare earth compound in which the amounts of these componentsand metals have been reduced; fluorinating the compound using a fluorinesource, such as hydrofluoric acid, ammonium fluoride or acidic ammoniumfluoride; heat-treating the fluorinated compound at a temperature of notmore than 400° C.; and pulverizing the heat-treated compound. Thecerium-based rare earth fluoride thus obtained desirably contains rareearth elements in a total amount of 60 to 75 mass % in terms of oxideand fluorine in an amount of 20 to 30 mass %. The term “cerium-based”used herein means that the content of cerium in the rare earth elementsis not less than 40 mass %, preferably not less than 60 mass %, in termsof oxide.

When the temperature at which the fluorinated compound is heat-treatedin the aforementioned process is higher than 400° C., reactivity of therare earth compounds, such as rare earth oxide, with the fluorinedecreases, producing hard aggregates during firing. The aggregates maybe abrasive grains that produce scratches. If such grains are contained,the polishing rate cannot be increased. Therefore, the heat treatmenttemperature has to be controlled to not more than 400° C.

In the present invention, the mixed light rare earth compound that is aprimary raw material is fired at a temperature of 500 to 1100° C. tothereby form mixed rare earth oxide. The thus-obtained mixed rare earthoxide and a cerium-based rare earth fluoride that is a secondary rawmaterial are mixed at a predetermined ratio, and the resultant mixtureis wet-micropulverized. The mixing ratio is appropriately determined inaccordance with the fluorine content to be required for the finalproduct (cerium-containing abrasive). Thus, in the present invention,the content of fluorine in the final product can be readily regulated ifthe mixing ratio of the cerium-based rare earth fluoride is modified.Pulverization is carried out by use of a medium mill, such as a wet ballmill. In the present invention, the average particle size of thepulverized product is preferably 0.5 to 3.0 μm.

Subsequently, slurry containing a mixture of the thus-pulverized mixedrare earth oxide and the cerium-based rare earth fluoride is dried andfired. The firing temperature is in the range of 600 to 1100° C.,preferably 800 to 1000° C. The fired product is subjected to cooling,disintegration and classification to thereby yield a cerium-containingabrasive. The abrasive preferably has an average particle size of 0.5 to3.0 μm and contains fluorine preferably in an amount of 1.0 to 10 mass%, preferably 5 to 10 mass %.

In the present invention, the mixed rare earth oxide obtained by firingthe mixed light rare earth compound at 500 to 1100° C. is preferablymixed before use with a mixed rare earth carbonate obtained bycarbonating a mixed light rare earth compound which has not been firedand with a cerium-based rare earth fluoride. In this case, fluorinecontained in the rare earth fluoride reacts with lanthanum contained inthe mixed rare earth oxide and mixed rare earth carbonate, yieldinglanthanum fluoride. Mixing the mixed rare earth carbonate with the mixedrare earth oxide can promote the reaction between fluorine and lanthanumto yield lanthanum fluoride.

Lanthanum oxide contained in an abrasive has strong basicity, causingplugging of a polishing pad during polishing. This adversely affectscirculation of aqueous abrasive slurry for refreshing the surface to bepolished. In particular, since a low-cerium-content abrasive has arelatively high lanthanum content, the above problem is likely to arise.In the present invention, since the mixed rare earth oxide is added withmixed rare earth carbonate and cerium-based rare earth fluoride and thensubjected to wet-pulverization, drying, firing, disintegration andclassification, lanthanum oxide is converted to lanthanum fluoride, thusenabling the aforementioned adverse effect to be suppressed duringpolishing.

The cerium-containing abrasive of the present invention is usedgenerally in the form of a powder. However, when used as an abrasive, itis generally transformed into an aqueous dispersion thereof and used forfinish-polishing a variety of glass materials and glass products, suchas glass substrates for optical lenses, for optical or magnetic disksand for liquid crystal displays.

Specifically, the abrasive is dispersed in a dispersion medium such aswater to thereby prepare approximately 5 to 30 mass % of slurry for use.Examples of the dispersion medium preferably used in the presentinvention include water and aqueous organic solvents. Specific examplesof the organic solvents include alcohols, polyhydric alcohols, acetoneand tetrahydrofuran. Generally, water is employed.

When objects such as glass substrates are polished by use of thecerium-containing abrasive of the present invention, no surface defects,such as pits and scratches, are generated, providing polished surfacesof excellent quality.

The present invention will next be described in detail by way ofexamples, which should not be construed as limiting the inventionthereto.

EXAMPLE 1

A rare earth concentrate (ore) containing 47 mass % of rare earthelements calculated in terms of oxide, 53 mass % of impurities otherthan the rare earth elements, 2 mass % of medium-to-heavy rare earthelements calculated in terms of oxide and 8 mass % of Nd calculated interms of oxide was treated to obtain a mixed light rare earth compoundin which the content of the impurities other than the rare earthelements was reduced to not more than 1 mass % and the contents of themedium-to-heavy rare earth elements and Nd were reduced respectively tonot more than 1 mass %. The compound was carbonated using ammoniumbicarbonate to obtain mixed rare earth carbonate.

The mixed rare earth carbonate thus produced contained 49 mass % of rareearth elements that contained 60 mass % of cerium, both calculated interms of oxide. Two liters of the mixed rare earth carbonate was firedin an electric furnace at 800° C. for 2 hours to obtain mixed rare earthoxide. To 1 kg of the mixed rare earth oxide, was added 300 g ofcerium-based rare earth fluoride (containing 27 mass % of fluorine and45 mass % of rare earth elements calculated in terms of oxide thatcontained 45 mass % of cerium calculated in terms of oxide) produced byadding fluoric acid to the mixed light rare earth compound andheat-treating the resultant mixture at 400° C. for 2 hours. Theresultant mixture was pulverized with a wet ball mill to obtain slurrythat contains particles having an average particle size (D50) of 1.5 μm.The slurry was dried, fired in an electrical furnace at 900° C. for 2hours and subjected to cooling, disintegration and classification toproduce a cerium-containing abrasive. The average particle size (D50)used herein means the particle size corresponding to a 50% cumulativevolume as determined from the particle size distribution curve andmeasured using a Coulter Multisizer (produced by Coulter KabushikiKaisha) equipped with a 30 μm-aperture tube.

The cerium-containing abrasive thus obtained was dispersed in water toobtain slurry having a concentration of 10 mass %. With the slurrypolishing liquid, alkaline-free glass plates for thin-film transistor(TFT) panels were polished under the following conditions, and thestates of the polished surfaces were evaluated.

Polishing conditions:

Polishing machine: 4-way double-sided polishing machine

Object to be processed: 5 cm-side alkaline-free glass plate with an areaof 25 cm²

Number of processed plates: 3 plates/batch×2 batches

Polishing pad: Foamed polyurethane pad (LP-77, product of Rhodes)

Number of revolution of the turntable: 90 rpm

Feed rate of slurry: 60 ml/min

Operation pressure: 156 g/cm²

Polishing time: 30 min.

Six alkaline-free glass plates for TFT panels were polished. Thethickness of each plate was measured, before and after polishing, atfour points by use of a micrometer. The measured values (4 points×6plates) were averaged, and the average thickness value was used as thepolishing rate (μm/min). In addition, the surface of each glass platewas visually observed using a halogen lamp of 200000 l× as a lightsource to count the number of scratches per polished surface. Thecentral-line surface roughness of each glass plate was measured by meansof Taly-step (product of Rank-Taylor Hobson).

The results obtained, including the average particle size (D50) of eachabrasive, polishing rate and central-line surface roughness (Ra), areshown in Table 1 below. In addition, the results of X-ray diffractionmeasurement are shown in FIG. 1, and the physical properties of theabrasive are shown in Table 2 below.

EXAMPLE 2

The procedure of Example 1 was repeated, except that 80 weight parts ofmixed rare earth carbonate obtained by carbonating an unfired mixedlight rare earth compound with ammonium bicarbonate were substituted for40 mass parts of the mixed rare earth oxide used as the primary rawmaterial, thereby forming a mixture of the mixed rare earth oxide andthe mixed rare earth carbonate to yield a cerium-containing abrasive.

Polishing was performed by use of the thus-obtained cerium-containingabrasive in a manner similar to that employed in Example 1, and thestate of the polished surface was evaluated. The polishingcharacteristics and physical properties of the abrasive are shownrespectively in Tables 1 and 2 below.

COMPARATIVE EXAMPLE 1

The procedure of Example 1 was repeated, except that the mixed rarecarbonate was fired at 1200° C., to thereby yield a cerium-containingabrasive.

In the same manner as in Example 1, polishing was performed using thethus-obtained cerium-containing abrasive, and the state of the polishedsurface was evaluated. The polishing characteristics and physicalproperties of the abrasive are shown respectively in Tables 1 and 2below, and the results of the X-ray diffraction measurement are shown inFIG. 2.

COMPARATIVE EXAMPLE 2

The procedure of Example 1 was repeated, except that cerium-based rareearth fluoride was heat-treated at 800° C., to thereby yield acerium-containing abrasive.

Polishing was performed by use of the thus-obtained cerium-containingabrasive in a manner similar to that employed in Example 1, and thestate of the polished surface was evaluated. The polishingcharacteristics and physical properties of the abrasive are shownrespectively in Tables 1 and 2 below.

TABLE 1 Scratches Surface Ave. particle Polishing rate (number/roughness Ra size D50 (μm) (μm/min) surface) (Å) Example 1 1.54 2.550.17 9 Example 2 1.46 2.68 0.08 8 Comp. 1.55 2.23 0.50 13 Ex. 1 Comp.1.63 2.12 0.83 15 Ex. 2

TABLE 2 Main peak Primary Relative Main peak intensity particle sizesurface area position (deg) ratio (nm) (m²/g) Example 1 28.5 0.52 30 4Example 2 28.5 0.45 40 3 Comp. Ex. 1 28.5 0.18 80 2 Comp. Ex. 2 28.10.30 70 1.6

As is clear from Tables 1 and 2 above, in Examples 1 and 2, there wereobtained cerium-containing abrasives that provided a high polishingrate, generated substantially no scratch on the polished surface of thealkaline-free glass and provided a high-quality polished surface havingsmall surface roughness.

In contrast, in Comparative Example 1, since the temperature at whichthe mixed light rare earth compound was fired was high, the reactionwith the added rare earth fluoride did not proceed sufficiently to allownon-reacted rare earth fluoride to remain. In addition, the polishingrate was high, and scratches were generated to increase the surfaceroughness.

In Comparative Example 2, the effect of enhancing the polishing rate waspoor, because of the large average particle size attributed to highheat-treatment temperature of the added mixed rare earth fluoride. Inaddition, scratches were generated to increase the surface roughness andimpair the quality of the polished surface.

INDUSTRIAL APPLICABILITY

Use of the cerium-containing abrasive according to the present inventioncan maintain a good polishing rate for a long period of time evenrelative to hard glass, and enables a polished object to have agood-quality surface with few scratches and small surface roughness.

1. A cerium-containing abrasive comprising cubic composite rare earthoxide and composite rare earth oxy-fluoride, wherein the abrasivecontains not less than 90 mass % of rare earth elements in terms ofoxide and the rare earth elements contain not less than 55 mass % ofcerium in terms of oxide.
 2. The abrasive according to claim 1, whereinthe abrasive, when subjected to X-ray diffraction measurement, has amain peak resulting from the cubic composite rare earth oxide that isnot less than 28.2 degrees at 2θ.
 3. The abrasive according to claim 1or claim 2, wherein the abrasive, when subjected to X-ray diffractionmeasurement, has an intensity ratio of a main peak resulting from therare earth oxy-fluoride to a main peak resulting from the cubiccomposite rare earth oxide of 0.2 to 1.0.
 4. The abrasive according toclaim 1, wherein the abrasive has primary particles with a particle sizeof 10 to 50 nm and a relative surface area of 2 to 10 m²/g.
 5. Theabrasive according to claim 1, wherein the abrasive contains 5 to 10mass % of fluorine.